A PDF file detailing an improved, non-toxic procedure for refining a cold-pressed linseed oil to make it non-yellowing and fast-drying is available here. This has been updated 1-16-14 and now includes five other traditional refining methods.

Avoid cheap, adulterated oil! It has to be pure. Look at the label, if it's got an anti-oxidant mix in it, it's a waste of time for this purpose.

This file is available in Russian here thanks to painter Mikhail Bulashov.

recreating the oil

Updated 2-13

The most fundamental technology lost from older painting is how early painters refined and modified their oil. Later, as commercial tube paint made with raw oil became more and more prevalent, so did various resin or resin and oil mediums designed to alter the behavior of this paint. "The Artist's Assistant" by Leslie Carlyle makes it clear that in 19th Century England, the search for the older manner of painting resulted in a virtual minefield of complex materials. However, the research carried out by the National Gallery and reported in their yearly Technical Bulletins consistently suggests that very little resin was ever used, that most of this was soft -- i.e. larch balsam -- and that the use was in "small amounts," possibly fused into the oil in some cases, and that the use of resin is never global until the 19th century. This is logical: older painting is consistent in its technical conservatism and economy of means.

Having made and worked with amber, copal, and sandarac varnish for several years, and being somewhat attached to this technology, this simplicity was at first hard to comprehend. How could it be done otherwise? What could "the oil" do? Ultimately, in 2007, I decided it would make the most sense to learn an all-oil method in order to compare the techniques fairly.

The results of this experience suggest that two fundamentally different systems exist. The original one is based on the behaviors and rheology of high quality, hand-refined oil. The modern one is based on the behavior and rheology of generally lower quality mechanically refined oil.

While the specific older system can never be known fully or with certainty, working with its principles via older recipes and procedures leaves no question that it offers a more functional and evolved way of utilizing the oil.

The older system naturally leads to ways of working that are unavailable to the modern one. This is because all the oils used by current manufacturers are refined according to standards for the food industry. The goal of this refining is to minimize oxidation that can cause rancidity. As such, without added driers, it is not possible to get a modern oil to dry quickly. This is not the case when beginning with an unrefined oil, where, without using lead or other metals, the potential for oxidation can be enhanced by various traditional refining methods.

In the modern system, the emphasis is on purchasing ready-made products. Science is often part of contemporary marketing strategies, but this is the selective science of the marketplace: the science of technical art history is more detached, and tells a very different story. Once upon a time, the art and craft informed one another in practice. Hand-refined oil still offers tremendous potential within a creative painting process.

A simple example of what became lost. Both of these bottles contain linseed oil. The one on the left was hand refined according to an ongoing recipe trail that begins in print in the 16th century. The one on the right was put up by a Well knowN international art materials company, probably in the 1960s, probably a hot-pressed, alkali-refined oil. The oil always darkens the most where it is not exposed to the light, under the cap. As you can see, one of these things is not like the other. Will age darken the oil on the left? No, darkening occurs as a result of the byproducts of the initial polymerization. This is why, when the varnish is removed, well-made older paintings such as Bacchus and Ariadne or The Night Watch look like they just left the easel. Cold-pressed, hand-refined linseed oil does not darken over time. This is why a company like David Davis in New York was able to sell a hand-washed, cold-pressed oil for so long, for so much money, to knowledgeable painters. These painters had experienced the alternative. Are things better now? Yes, but there's still oil being used in paint by some manufacturers that gets quite dark, and no modern oil dries well on its own. Can you buy an oil like the one on the left, which dries overnight and doesn't yellow? No, no one would pay for it, it has to be handmade. And if you make it, you will not only have a great oil to work with, but a vital clue about what has been lost.

overview

Like many painters I first became concerned about the commercial version of linseed oil by observing it in action. The De Mayerne manuscript (1620 --1632) is full of recipes for fixing linseed oil so it won't yellow as much: this was obviously an issue with the oil that was still often preferred for its strength and quicker drying. This, in addition to the color of the oil I saw oozing out of tubes of commercial paint, even very high quality commercial paint, always made me nervous about using commercial, "alkali-refined" linseed oil. I didn't doubt that linseed oil could be used in a non-yellowing manner: this was clear from older paintings and the analyzes by the National Gallery in their yearly Technical Bulletins. In fact, commercial linseed oil has gotten better over the years; several manufacturers now sell a cold-pressed oil which, while ultra-refined, in fact doesn't yellow. However, it has been so cleansed, so stripped, that it's also devoid of all character, has none of the snap or pop associated with OM linseed oil. Whether this is important or not depends on how relevant the older craft seems as a foundation. For smooth surface work on stretched canvas, a quality modern oil is fine. But the older craft begins with processing the oil, and this leads to places modern oil cannot.

Painters are fortunate now to have available several different choices in cold-pressed, unrefined, organic linseed oil. Oil of this quality was literally unavailable to 20th Century writers such as Taubes, Mayer, Laurie, or Wehlte, and it is important to view almost all written comments on "linseed oil" from this perspective, as this is always the commercial linseed oil of the period. There is no discussion of refining the oil in any 20th century text I have encountered, except Carlyle's Molart Fellowship, Historical reconstructions of artist's oil paint: an investigation of oil processing methods and the use of medium modifiers (Canadian Conservation Institute, 2000). Here Carlyle began with flax seed, and pressed it by hand before washing it. But this is in the context of technical art history, not a painting manual for general student use.

My first experience refining linseed oil followed the guidelines in Eastlake and Merrifield, washing the oil using water, sand, and salt. It took six washings of one week each to remove the majority of the fatty acids. The washing was done using the cold-pressed, organic oil Allback oil from Sweden. Before and after photo above, lighter is after. For more on this process, which did result in a much quicker drying oil which yellowed to about the level of refined walnut oil, even less after aging a year in the light, see refining section below.

Simply buying the highest quality oil is not enough, it still must be refined to remove its various impurities that lead to yellowing. This process should not to be confused with the various other, quicker processes -- breadcrumbs, lavender, alcohol, et al -- simply designed to clear a cloudy, absolutely unprocessed oil, typically found now in a health food store. All of these methods have their origin in the older authors, from De Mayerne to Pacheco. While these "quick and simple" methods may lead to a relatively non-yellowing oil, the added plus of the older procedure is an oil that dries much more quickly, and, for this reason, creates a system of its own in practice.

Oil was often used to make paint in a somewhat heat-bodied state: see below and article on Van Dyck in National Gallery Bulletin 19. There are many subtleties here, especially in terms of lower temperatures, and this provides a logical, inexpensive way of increasing the gloss and elastic working characteristics of the paint in a thin layer while shortening the drying time. Between the almost watery thinness of raw oil and the too great viscosity of sun or stand oil lies a world of interesting possibilities which it seems older painters explored fully. While much of the early work I did with the materials concentrated on the potential of a hard resin varnish in the medium or paint, it was in fact the changes in paint rheology observed by using slightly heat-bodied oil to make paint which suggested that the medium was not as focal or as large a factor in the system when the paint itself has been effectively "resinated", giving it an elastic and prehensile working quality that is essentially the opposite of the false-gel "delicious butteriness" of commercial tube paint.

Above, chart of a commercial linseed oil refining process, from Scaldis in Belgium. In commercial oil, everything is removed. This means that the oil has a long commercial shelf life, and can result in a non-yellowing oil, but, from the perspective of both drying time and working character, this is overkill. The oil is not refined to enhance oxidation, but to retard it.

quality linseed oils

Allback markets a cold-pressed, organic linseed oil grown in Sweden, a country with exactly the right climate for producing high quality linseed oil. They also market paint for the exterior and interior of houses using this oil and pigment according to old Swedish recipes that they've researched. This is a high quality product. The nutritional oils below are even better, but they are also more costly unless buying in bulk. The Allback website is here.

A very high quality, reasonably priced oil is put out by Azure Farms in Oregon. This oil is very alkaline, smells like a field of flowers.

A more readily available retail nutritional oil in America that has worked out well in use is the Flora brand from British Columbia. Expensive retail at 20 dollars a pint, available online at 25 dollars a quart. Perhaps overkill, but very carefully processed, 50C maximum temperature, performs well in all the refining methods here. These oils are being augmented with an increasing variety of ingredients. When shopping for this type of oil, be careful to get "plain", not high-lignin, lemon-flavored, strawberry shake, etc.

Another high quality oil which is available cheaply in the USA is the Jarrow brand, organic and cold-pressed, packed under nitrogen in Washington State. Available for between 16 and 18 dollars a quart online.

On the other hand, this oil by Omega Nutrition has been reported to take a very long time to dry when refined. So this might be one to avoid for this purpose.

Another less expensive flax oil that someone had issues with. This one contains an "anti-oxidant mix" that includes sunflower oil. You can look for less expensive oil, but make sure it's JUST oil.

Very high quality oil for processing is also available from Jedwards in Boston. The five gallon price is especially attractive. Jedwards also has a refined cold-pressed organic oil at a very good price. This oil does not dry quickly, but it is non-yellowing. Sometimes this oil is from China, I haven't used this but have doubts about things from China in general, you might get a smaller amount of this first to see what it is like.

refining linseed oil

In 2006 I began working with the organic cold pressed linseed oil from Sweden marketed by Allback. I refined it several traditional ways in the hope of getting a fast drying oil that didn't yellow.

The first experiment was washing the oil with salt, sand, and water, a technique spoken of in many older books. This took six weeks to do, and a significant portion of the oil was lost, about a third. This process is not to be confused with the various quicker processes designed to clear a cloudy, totally unprocessed oil. Oil can also be made non-yellowing simply by traditional exposure to the sun in full glass jars, but this takes several years.

The refined oil does dry much faster than the unrefined oil, but it also yellowed slightly at first. Much less than the alkali refined oil of commerce, a little more than walnut oil. After aging in a sunny window for a year, this oil was lighter and yellowed still less, the same amount as the better quality commercial linseed oils in tests – very little. I like the slightly grippier, tighter working quality of the salt-refined oil better, but this may not be of interest for larger and smoother painting styles. I also noticed that this oil did not dry from the surface or form a skin, in spite of drying quickly, one and a half to two days. Alla prima work made on a summer morning has been dry before nightfall.

Is this process worth the effort? As with many things in painting, it depends. Salt refined linseed oil has a unique working character that helped me understand, for the first time, why linseed oil could be preferred over walnut oil by older painters. So, washing with sand and salt, a traditional method documented as far back as the 17th century De Mayerne manuscript, offers an avenue to an oil of this type. Are there other high quality linseed oils available commercially that require no further processing? Yes. But, because of the way they are processed – a process designed for edible oils, not oils for painting – they don't dry quickly, therefore, the system the salt-refined oil creates naturally on the palette cannot occur. This becomes especially relevant in terms of further modifications of the oil, or using the putty method to paint without solvents.

A combination of several of the traditionally known factors contribute to an oil which dries faster, harder, and with less yellowing. These factors are: a high quality cold-pressed oil to begin with; refining using a traditional method, subsequent aging or preheating of the oil, and use of the oil in thin layers which will dry quickly. The one exception to the thin rule seems to be the chalk putty medium, which does not increase yellowing even when used thickly. More information on using the putty medium can be found here.

mucilage

Linseed oil contains a tremendous amount of water soluble mucilage that is transferred to the oil in any modern processing method. This is the mucilage generated by boiling 60 cc (1/4 cup) of seeds in 475 ml (2 cups) of water for five minutes. The temperature of boiling water is also the temperature allowed for "cold-pressed" oil. It is dense and gluey stuff, remains water-soluble, and darkens to deep brown over time when exposed to the light (photo of aged mucilage embedded in recovered refining sand below).

science and the washing method

Research has established that in a water-washed oil the fatty acids have been altered in three ways: by increased oxygenation, by increased dimerization (molecular linking), and by cis-trans isomerization (meaning that large molecules that were bent become straight, and therefore easier to link when polymerizing). These factors add up to an oil that is much more readily polymerized, i.e., dries more quickly, about 2.5 times faster than raw linseed oil. In other words, the efficacy of the traditional washing method, dating to the 17th century in print, is confirmed by the findings of current research. More on the science of this in explained in Effects of traditional processing methods of linseed oil on the composition of its triaglycerols (2004) by Jorrit D.J. Van den Berg, Nicoletta D. Vermist, Leslie Carlyle, Michal Holčapek, Jaap J. Boon. Aspects of this are relatively accessible, notably Dr. Carlyle's seminal contribution, and it can be downloaded free online.

ethanol and the refining process

Ethanol surfaces in the refining process in the well-known entry from Van Dyck in the De Mayerne Manuscript. Recently, ethanol has been established by research as a non-invasive way of removing the mucilage, phospholipids, and free fatty acids from an edible oil. Doing this is simple: a volume of oil is shaken repeatedly for a day with approximately the same volume of 35% ethanol. This is about 8 oz. of the cheapest "vodka" with 1 oz. water added. At the end of the day, water at three times the volume of the oil and ethanol is added. The next morning, the oil will have separated, with an opaque white mixture of water and ethanol beneath it. It can then be processed further as desired. But, it is important to note that, at this point, the oil will still take almost as long to polymerize as the raw oil. So, while ethanol provides a way of removing various impurities from the oil, it does not replace the washing process, aging in the light, or preheating as methods of creating a higher quality, faster-drying oil. It may provide a quality beginning, but it does not provide an analogous end product.

The yellowing -- orange-ing? -- of linseed oil in several artist grade commercial paints. Significantly less yellowing in paint made by Robert Doak or Blue Ridge in Asheville on the right for comparison. If you are painting in midtone oriented colour, or using transparency in the manner of older painting, the most non-yellowing oil is slightly important.

Another example of yellowing in a commercial paint. Old Holland makes a big deal out of their oil that is cold-pressed in windmills. Then again, they also claim to go back to Vermeer when in fact they were started in the 80's. And "cold-pressed" in this context means at 100C or below: not that cold! Years ago I had some issues with OH paint yellowing, such as this example here of manganese blue, and always thought it was because the vaunted cold-pressed oil was unrefined, i.e., full of mucilage that would attract water and darken. Now, manganese blue is a colour that may darken anyway, simply due to the manganese content, so it's not the clearest example, but it's the only OH colour I have at this point. So, I'm going to do a more formal test, the colour itself, then the colour cut with a rich chalk putty made with hand-refined oil as an attempt to preserve the original colour.

It's important to do yellowing tests of the oils, it takes about three or four months to find out what is going to happen. I try to use the same ground I'll be working on, in this case a glue gesso. Yellowing tests are very dependent on relative humidity. Oils will yellow more with high humidity, less with low. Oils that have yellowed in high humidity will become less yellow in low humidity. Oil needs light to dry without yellowing, one thing all the older texts agree on its that paintings need to dry in the light. This does not mean blazing summer sun, a north window is adequate.

improved water, sand, and salt method

For several years I worked on methods of shortening the Eastlake procedure, and eventually was able to wash the oil three times with one rinse and a slight pre-heating to clear it. This procedure, along with other traditional refining methods, is detailed in a PDF file downloadable here. The procedure is outlined in photos below.

The procedure is sensitive to the type and quantity of ions in the water. It may be necessary to cut very hard water with distilled or spring water to avoid oil being trapped in a layer of "break," as in the image below. Trapped oil does emerge over time, so this break can be saved and recycled, but the ideal process clears quickly with no entrained oil.

This is the current oil system I use with the putty, everything is based on the hot salt refined organic linseed oil. This actually dries too quickly in the summer for some things, and I use a water refined oil, but in the winter it's just about right. I don't use the raw oil on the left at this point except in underpaintings and paint. The ones that are getting the most use are the two in the center, these are relatively new additions over the last few months. I used to use the thicker oils in small amounts to modify the thinner oils, but these medium density oils are much easier to control.

Yellowing test (January 2010) of various organic cold-pressed linseed oils refined by the methods here. These oils have all remained close to colourless over a period of several years.

aerated oil

The oil can be thickened to any extent in a glass baking tray in a thin layer, the speed of this is dependent on ambient temperature. It can also be thickened to some extent by oxygenating it using an aquarium pump set up with an airstone. A pump for a forty gallon tank will do at least 4 half gallon jars. These pumps are often advertized as "silent," just as commercial mediums are advertized as "non-yellowing." So if you are at all sensitive to noise, this needs to be set up in a secluded location, with the pump further insulated against sound. Different types of oil will thicken at different rates, this is also dependent on ambient temperature. One to two weeks will usually thicken the oil enough to create a saturated paint film when used as a medium.

microemulsions

Once the oil is washed, it is then dried to remove all traces of water. But one of the things I've been wondering about is the possibility of long term refining in the light with this small amount of water remaining in the oil. So, the water, its ions, and the oil are activated by the light to continue to refine it. In this experiment, several clouds of precipitate have been formed. They go to the bottom of the jar, but then, because the water remains in the system, another cloud eventually forms. This system is called a microemulsion: a very small amount of one ingredient – the water – is held in suspension in a much larger proportion of another ingredient – the oil. So, I'm wondering if this method-within-a-method may have accidentally have become part of some older oil processing techniques.

the colour of the oil

One of the things that is perennially confusing is the colour of the oil. Everybody likes pale oil! But, this doesn't necessarily mean anything. An oil can be made colourless with bleaching clay, as is typically done with commercial cooking oils and many commercial painting oils. But, if the impurities remain in the oil – mucilage, phospholipids, free fatty acids – guess how effective that process is? Exactly. The original colour of the oil is fugitive, it is the colour the oil turns once it has polymerized that is important. So, with that in mind, here are some oils. Number one is the Jedwards refined organic linseed oil. This is probably the highest quality refined oil on the American market at this point, but it doesn't dry quickly because the refining process itself is designed for an edible oil. But this is not unique, there is, to my knowledge, no oil on the market that is refined any other way except the Allback boiled oil, and this contains a manganese drier. So, with the Jedwards refined oil, I heat it for an hour to 100C to make bouncy handmade paint with it, and am also thickening it in half full jars like this one. The process is slow, this is a little lighter and thicker after three months, but by next summer this will be colourless and quite thick. If you do this, make sure to visit the jar every now and then and open it, as otherwise it can become sealed quite tightly by the vacuum created by the oil absorbing oxygen. Wehlte writes that a jar can implode. I've never had this happen, but I have had to puncture lids to get jars open.

Oil number two is a vodka refined oil from last May that was then washed for two weeks in spring water. I tried really cheap vodka for this but found that cheap real vodka worked better. The ethanol washing process produces a slow-drying oil with less bounce, and a generally finer working character. A variation of this process using egg yolk as well was given by Van Dyck to DeMayerne. So, even though this oil has been open for seven months and has thickened, it is still somewhat mobile. Oil three, below it, is an oil that was washed for three weeks with spring water in the light. This cleans the oil well but does not produce much of an increase in drying time. This oil was then heated with ground calcite for four hours to 100C, a very reliable way to get both a cleaner and somewhat faster drying oil. This oil has not been exposed to oxygen, and is in a wider jar, yet it is as light as jar number one. So, that's probably good. Oil number four is the Spectrum Naturals walnut oil available in larger grocery stores in America. In theory this has been refined, but it can be made cleaner. This was washed with salt and allowed to sit on a sunny windowsill for several weeks, then put through the same process a second time before rinsing with copious pure water. It dries twice as fast as the raw oil, but really, that's not that fast. But it is very clean, seems to glow from within, and would be a good choice for the crimson, pure blue and cool green colour scheme like the one Perugino developed where yellowing is out of the question.

walnut oil

There are several types of walnut oil, from the ultra-refined oil sold by Kremer to the mechanically refined but cold-pressed Spectrum Naturals oil, to the unrefined Kremer or Loriva brand oil. The Loriva oil has recently been altered to include "walnut flavor concentrate" and should probably be avoided for this reason. There are also relatively expensive walnut oils from France or England, good if you can find them in a dented can store, but beware of any oil containing a preservative, vitamin E, as this will retard drying completely. I work with the walnut oil from Jedwards now, and wash it. For the most part I use this oil in an "aged" or possibly "semi heat-bodied" state, see below, another older technology which is simple but helpful: makes for less yellowing and a faster-drying paint with more spring or "boing".

pre-heated oil: 48:230

Typically, oil is heat-bodied by "fuming," heating just to the point of smoking, for a period of up to an hour. This produces a darker and much more resinated oil which is somewhat slippery, though still more fluid than sun oil or stand oil. Using the repeated preference in older texts for "old" oil as a clue, I found that by lowering the temperature to around 230 degrees Fahrenheit (100C, the boiling point of water) another level of possibilities emerged as the oil at this temperature doesn't darken and becomes only slightly thicker. This is an area where a great deal of latitude exists: I've experimented with heating for as long as ten days, but the time that I work with most in making paint is 48 hours, mixing this with raw oil in different proportions. This 48:230 oil is also a good foundation for mediums. It is possible, if no means of maintaining a low temperature over this long period exists, to raise the temperature to 300 Fahrenheit (about 150C) and stir the oil for an hour or so. This is still well under the temperature (280C -- 536F) at which the triglycerides begins to break down.

the paint

Paint made with preheated walnut oil and raw sienna, no other additions. This paint is inherently thicker and more bouncy, without becoming glutinous or stringy. This approach offers many fascinating rheological possibilities using the standard array of inert additions and medium ingredients. It was interesting to note this paint's increased sensitivity to the introduction of a hard resin varnish. While amber, copal, or sandarac will cause most commercial tube paint to become variously more thixotropic, this characteristic of seizing or gelling becomes significantly more pronounced and reliable in homemade paint. When using a preheated oil paint any hard-resin addition must be measured very carefully as the paint becomes so immobile that solvent or more oil must be added at very small concentrations. Eastlake gives very early evidence from the Strassbourg MS that paint was made with oil that had been boiled briefly with a drier, in this case calcined bones and calcined zinc sulphate. This same passage (15th century, information from 14th century) also instructs that the paint is to mixed with a few drops of varnish on the palette. There are many other examples from older texts of oil heated with or without driers prior to use. This oil holds the pigment in suspension better than raw oil, and the double-carbon bonds between the triglycerides formed by the heat create a different type of ultimate polymerization than that of raw oil: atmospheric oxygen is less involved, resulting in a decreased potential for non-reversible yellowing in the paint film by hydroperoxides. So, whatever purification process is or isn't used on cold-pressed, unrefined linseed/walnut oil, a final stage of heat prepolymerization seems potentially as important in attempting to minimize long term yellowing. In my experience this oil has been superior in every way to raw oil -- although I haven't experienced raw oil aged more than a year, an item older painters may very well have had access to.

This oil can most effectively be made using a magnetic laboratory hotplate, but can also be made using a small deep-fryer with a thermostat or, hotter but still acceptable, the small Crockpot made for heating "party dip". This oil also became the foundation of the chalk and oil putty mediums, with small amounts of thicker oils in various recipes. All these preheated oils dry harder, more quickly, and more stably than raw oil without the deeper saturation, shine, and dramatic change in rheology of fumed or sun oil.

Mixture of Burnt Green Earth and Dark Ochre made with preheated walnut oil and nothing else. Dense but mobile paint like this cannot be made on modern machines without significant additives. It can be made in the studio by simply preheating the oil. When older painters talk longingly of older paint, it is because it was made with more pigment. Even high quality paint from the 1950's -- Lefebvre-Foinet, or Windsor and Newton in the old style metal capped tubes -- has this more dense pigmentation.

the quote

"Heat pre-polymerization has several effects on the oil. Drying properties are improved and are further enhanced by the addition of metal salts (usually those of lead) during the process. The refractive index of the oil is increased, thus reducing light scattering at the pigment-medium interface and thereby increasing the saturation of the pigment colour; the paint film may also have a glossier appearance. The pigment is less liable to sink in the oil film, which itself decreases less in volume than a conventional oil film, reducing the amount of wrinkling that may occur. White paints appear less discoloured because, as the polyunsaturated fatty acids initially present in the paint film are destroyed by the formation of carbon-carbon single bonds, there is less scope for the formation of chromophoric and auxochemic groups, the presence of which give the yellow appearance to the film." From Rembrandt and his Circle: Seventeenth-Century Dutch Paint Media Re-Examined by Raymond White and Jo Kirby. National Gallery Technical Bulletin volume 15.

It would be logical for an oil rich in carbon-carbon bonds to also be more resistant to subsequent oxidation of the paint film. But there is of course some question as to just what "heat pre-polymerization" means in this case.

painter’s paint

Using painter-refined, cold-pressed organic linseed oil, many pigments available now can simply be mixed in needed amounts on the palette. While not quite as saturated or fine as paint made by mulling, this is a good way to begin to understand the handmade paint process. Pictured, Burgundy Red Ochre Deep, No. 11576, available from Kremer Pigments.

The paint is naturally somewhat dense and stringy.

Separating the paint before mixing with a chalk putty medium.

When mixed with a chalk putty medium, it becomes lighter, but also brighter.

If a denser texture is needed, more chalk can be added.

Many different values and rheologies are possible without resorting to white or a medium containing resin or solvent.

painting oil

This oil can be used to thin the paint
in the later layers, is perhaps not rich enough to finish a painting, but close. It's composed of one part Eminent Oil and three
parts oil which has been heated to 150 C for four hours.
The two oils are mixed thoroughly and given ten minutes in a boiling
waterbath. This oil is relatively thin but somewhat adhesive, it
doesn't run. It becomes increasingly thixotropic and loses all color on exposure to oxygen.

fumed silica gel

Fumed, pyrogenic, or colloidal silica is a pure form of silicon dioxide made at very high temperatures with various industrial uses. It is chemically the same material as quartz. When oil is combined with this material it forms a gel. Various viscosities of oil can be used and various ultimate viscosities of gel can be achieved. Works well for both lively alla prima work and as a thin couch in later layers in more formal painting. The key here is knowing the behavior of your oils well and using as little of this medium as will do the job effectively. Vastly preferable to the fragile mastic gel mediums, can be made to perform in a similar way. Fumed silica is very light and needs to be handled with care using a respirator. The gel can be made in bulk and stored in tubes or wrapped airtight in aluminum foil. If you make this, try some small studies and tests first to get an idea of how it handles before making larger amounts. This material needs to be made with an oil component that dries well. Fumed silica is very light and responsive to static electricity, great care should always be used opening the tub. It is a respiratory hazard and should only be handled in the dry state wearing a quality particle mask.

sun oil

At the far end of the thickness spectrum is the sun oil of the older painters, can be made with any drying oil. This should be a cold-pressed oil, it should be mechanically refined or washed to remove the fatty acid break but should not be the alkali refined commercial oil. I've had experience of sun oil made with unrefined oil darkening. You need a hot dry climate to do this efficiently. The oil goes into a glass tray, a lid of glass goes on top but with an air space for ventilation. Several older sources talk about doing this in a lead tray or adding pieces of lead or adding a small amount of lead carbonate. This is a faster process but also results in a somewhat different material. Stir it now and then to make sure no skin is forming on top. Or allow one to form that is thick, and simply cut through it at the end. Heat gently to thin at the end in order to strain out bugs and other foreign matter. This takes about a month in a good climate, longer with less sun. However, even two weeks in August in Vermont, while not resulting in a particularly thick oil, does result in a pronounced increase in gloss and decrease in drying time. If you live in a more ideal climate don't let it get too thick as it continues to polymerize after being removed from the sun. A small amount of this is useful in mediums and is vastly superior to commercial stand oil, which dries slowly and can yellow badly. Sun oil is lighter and more sticky than fumed oil.

sun spike

Later in his career, Lord Leighton stopped using Roberson's medium and began using Bell's medium, made from blown oil and spike lavender. A perhaps more high quality version would be to use sun oil for the blown oil. A good place to start with this for use in a medium would be one part spike lavender and one part sun oil, using about 25 percent of this mix with other, thinner oil(s). For use alone, sparingly, two parts spike to one of sun oil could be used over a developed underpainting, or three spike to one sun for more liberal use. In finishing, the one to one mixture is fine, but should again be used sparingly. This has an open time of about half an hour, then sets progressively like a resinous medium. Spike lavender is very penetrating, and should always be used with ventilation. In the yellowing tests I've conducted, this medium yellows minimally. Should be kept in amber glass, small bottles.

stand oil

Stand oil is thickened at high temperatures in an atmosphere devoid of oxygen, making it thick and viscous, leveling and saturating in small amounts a medium. Many stand oils are made from relatively low quality oil since the process tends to make the oil less yellowing. These oils can yellow over time. There is one stand oil, though, that is made from high quality oil, the Kremer 73201, and this is a really nice material, quick drying and non-yellowing.

burnt plate oil

The original burnt plate oil was made by heating linseed oil, with or without a metallic drier, until it combusted, stirring until the material achieved a highly stringy and elastic quality. This material has been proposed by conservators Sarah Belchetz-Swenson and Phoebe Dent Weil as the material used by Rembrandt to make specialty impasto white: the process is illustrated here. While the extensive use of heat-bodied oil is not suggested by the research done by London's National Gallery and documented in their book, "Rembrandt: Art in the Making", this is nonetheless an intriguing material, in that the process results in a very long, highly saturating oil which is not sticky or glutinous like stand oil or sun oil. Graphic Chemical Company makes a line of linseed oils in various viscosities that have been cooked in a vacuum kettle at hight heat, with the volatile elements being progressively sparked off. A very small amount of one of these oils in a medium changes both rheology and drying time significantly. Oil #3 is thinner than stand oil, and has a bit of color. Oil #5 is about like stand oil, but is very pale. Oil #7, illustrated, is thicker than stand oil and is also a pale yellow. Oil #8 is like cold molasses or treacle, and has a light caramel color. These oils seem to dry to their color with minimum subsequent yellowing. However, they do take a long time to dry and increase the drying time of a medium. I've used very small amounts of #5 in a finishing medium, and have incorporated both #5 and #7 in small amounts into putty mediums. These oils in a medium produce paint which is ductile and blendable while retaining body and with significantly enhanced saturation. As an inexpensive commercial product available by the gallon, I was highly skeptical of this material, but it has proved itself in practice to be useful in very small amounts.

the putty medium

The putty medium is simple but can be configured in many ways. It will make paint which is thin and smooth, paint which is rough and broken, and everything in-between. It came about from studying the book "Rembrandt: Art in the Making" by the National Gallery Staff, in which they site research at a molecular level which determined that Rembrandt principally used linseed oil and chalk as the additions to the pigment. Paintings on this site since 2007 were executed in variations of the putty medium. More putty process details to be found here. More putty history and instructional text to be found here.